Tasks should accept rerunIfModified only run if a file was modified since task execution

[farnoy]

There's a common pattern with language-specific package managers only needing to re-run if I changed something like Cargo.toml, or if I checked out another revision and my Cargo.lock was updated. It's already possible to implement this with the status command, but I feel like this should come built-in with devenv.

tasks.cargo = {
  exec = "cargo fetch";
  rerunIfModified = [ "Cargo.toml" "Cargo.lock" ];
};

[sandydoo]

Related: https://github.com/cachix/devenv/issues/1616